This application claims the priority of Japanese Patent Applications No. 2001-297927 filed on Sep. 27, 2001 and No. 2002-259396 filed on Sep. 4, 2002, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light-emitting device and a method for manufacturing thereof.
2. Related Art
A light-emitting device whose light emitting layer section is composed of (AlxGa1-x)yIn1-yP alloy (where 0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61, which may simply be expressed as AlGaInP alloy, or more simply as AlGaInP, hereinafter) can be provided as a high-luminance device when it employs a double heterostructure in which a thin AlGaInP active layer is placed between an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer, both of which having a larger band gap than that of the AlGaInP active layer. Recent efforts have also succeeded in putting a blue light-emitting device into practical use, which device having formed therein a similar double heterostructure using InxGayAl1-x-yN (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and x+yxe2x89xa61).
Referring now to an AlGaInP light-emitting device, a light emitting layer section thereof having the double heterostructure is formed by stacking an n-type GaAs buffer layer, an n-type AlGaInP cladding layer, an AlGaInP active layer and a p-type AlGaInP cladding layer, all of which layers are grown in this order on an n-type GaAs substrate by hetero epitaxial growth process. Current supply to the light emitting layer section is effected through metal electrodes formed on the surface of the device. The metal electrodes are typically formed so as to cover only a center portion of the main surface of the light emitting layer section since it can otherwise serve as a light interceptor, which allows the light to be extracted from the peripheral area having no electrode formed therein.
An area of the metal electrode as small as possible in this case can ensure a larger area for light leakage around the electrode, which is advantageous in that improving the light extraction efficiency. Previous efforts have been made in increasing the amount of extracted light by modifying shape of the electrode so as to effectively spread electric current throughout the device. This strategy is, however, still suffering from an inevitable problem of increasing area of the electrode, which raises a dilemma such that decreased area for light leakage undesirably limits the amount of extracted light. There is now another proposal of raising the light extraction efficiency by covering the main surface of the light emitting layer section with an ITO (indium tin oxide) electrode layer having a high conductivity in place of using the metal electrodes, which is typically disclosed in Japanese Laid-Open Patent Publication No. 6-188455 or No. 1-225178.
Investigations by the present inventors, however, revealed that contact resistance with a compound semiconductor layer on the device side tends to become high by using the ITO-made transparent electrode layer as it is, which inevitably degrades the emission efficiency due to increase in series resistance. One typical method to reduce contact resistance is proposed in Japanese Laid-Open Patent Publication No. 1-225178, according to which an electrode contact layer composed of an InGaAs layer is provided between the ITO electrode layer and a semiconductor layer on the device side so as to be corresponded to the entire surface of the ITO electrode layer. It is, however, essential for this case that the electrode contact layer is made of InGaAs or the like having a low band gap energy in order to ensure ohmic contact, so that even an extremely small thickness thereof will inevitably result in degradation in the light extraction efficiency due to absorption of the light. Even for the case where the transparent electrode is used, a problem will still remain in a phase of manufacturing devices in that a metal bonding pad to which a wire for current supply is bonded must be arranged on the transparent electrode. This, however, tends to concentrate drive voltage to the areas where the highly-conductive metal bonding pad is formed, and tends to lower the light extraction efficiency due to poor current supply in the area around the pad, which serves as a light extraction area, so that it may not be always sure that using the transparent electrode promises effects to a sufficient degree.
Accordingly, the present invention is to provide a light-emitting device having an oxide transparent electrode layer as an electrode for driving light emission, and being capable of enhancing effect of improving the light extraction efficiency exhibited by such oxide transparent electrode layer, and also is to provide a method for manufacturing such light-emitting device.
The light-emitting device of the present invention premises that it has a light emitting layer section which comprises a compound semiconductor layer, and an oxide transparent electrode layer for applying drive voltage for light emission to the light emitting layer section, and that it is composed so that the light from the light emitting layer section can be extracted through the oxide transparent electrode layer, where a feature of the device resides in that an electrode contact layer for reducing contact resistance of the oxide transparent electrode layer is arranged between the light emitting layer section and the oxide transparent electrode layer so as to contact with such oxide transparent electrode layer, where on a contacting interface of such oxide transparent electrode layer, occupied areas and unoccupied areas for the electrode contact layer are arranged in a mixed manner. The electrode contact layer preferably comprises a compound semiconductor.
As has been described in the above, an oxide transparent electrode layer typically composed of ITO cannot always ensure a desirable ohmic contact even though a trial is made on bringing such layer into direct contact with a compound semiconductor layer on the device side, which may result in degraded emission efficiency due to increased series resistance based on the contact resistance. Whereas, the light-emitting device of the present invention is successful in reducing contact resistance of the oxide transparent electrode layer by placing the electrode contact layer for reducing contact resistance of the oxide transparent electrode layer so as to be brought into contact with the device side of such oxide transparent electrode layer. Further, the occupied areas and unoccupied areas for the electrode contact layer are arranged in a mixed manner on the contacting interface of the oxide transparent electrode layer, so that the light absorption by the electrode contact layer can successfully be reduced even when such electrode contact layer is, by nature, very likely to absorb the light from the light emitting layer section, since the light generated just under the occupied area for the electrode contact layer can leak through the non-occupied area adjacent thereto. Such formation of the electrode contact layer can successfully raise the light extraction efficiency of the device as a whole.
The electrode contact layer formed so as to cover the entire portion of the contact plane on the device side of the oxide transparent electrode layer will, however, result in the problems below:
(1) contact resistance of the oxide transparent electrode layer is reduced even in an area just under the bonding pad used for wire bonding, but this undesirably tends to concentrate the drive current, and consequently light emission, within such area, where much portion of the emitted light is shielded by the bonding pad and thus light extraction efficiency will be degraded; and
(2) the electrode contact layer may serve as a light absorber depending on material species of compound semiconductor used therefor, which will similarly result in degradation of the light extraction efficiency.
To solve these problems, a feature of the light-emitting device according to a first aspect of the present invention resides in that the contacting interface of the oxide transparent electrode layer has a first zone which comprises an area just under a bonding pad placed on such oxide transparent electrode layer and a second zone which comprises the residual area therearound, where the second zone is larger in the amount of extracted light than the first zone, and the electrode contact layer is formed with a larger ratio of occupied area in the second zone than in the first zone.
According to such constitution, ratio of occupied area of the electrode contact layer formed on the contacting interface of the oxide transparent electrode layer is smaller in the area (first zone) just under the bonding pad, which extracts a less amount of light, than in the residual area (second zone) which extracts a larger amount of extracted light, so that the first zone will have an increased contact resistance of the oxide transparent electrode layer. This resultantly increases a component of the drive current for the light-emitting device, which flows into the second zone while bypassing the first zone, and successfully enhances the light extraction efficiency to a significant degree. It is now preferable in view of increasing the light extraction efficiency that the drive current for light emission does not, as possible, flow through the first zone which extracts a less amount of light. It is therefore preferable that the first zone has formed therein no electrode contact layer as possible. It is also preferable that the occupied areas and unoccupied areas for the electrode contact layer are arranged in a mixed manner at least in the second zone in the contacting interface of the oxide transparent electrode, which second zone extracts larger amount of light.
The light-emitting device according to a second aspect of the present invention premises that it has a light emitting layer section which comprises a compound semiconductor layer and an oxide transparent electrode layer for applying drive voltage for light emission to the light emitting layer section, and that the device is composed so that the light from the light emitting layer section can be extracted through the oxide transparent electrode layer, where a feature of the device resides in that an electrode contact layer, composed of a compound semiconductor, for reducing contact resistance of the oxide transparent electrode layer is arranged between the light emitting layer section and the oxide transparent electrode layer so as to contact with such oxide transparent electrode layer; the contacting interface of the oxide transparent electrode layer has a first zone which comprises an area just under bonding pads and a second zone which comprises the residual area, where the second zone is larger in the amount of extracted light than the first zone; and at least the second zone has formed therein the occupied areas and unoccupied areas for the electrode contact layer arranged in a mixed manner. The occupied areas for the electrode contact layer are preferable to be formed in a discrete manner.
According to such constitution, even if the electrode contact layer formed in order to reduce contact resistance of the oxide transparent electrode layer is very likely to absorb the light from the light emitting layer section, the light generated just under the occupied area for the electrode contact layer can leak through the adjacent non-occupied area, so that light absorption by the electrode contact layer is avoidable. This desirably enhances of the light extraction efficiency of the device as a whole.
Next discussion relates to the electrode contact layer, which electrode is excellent in reducing effect of contact resistance of the oxide transparent electrode layer and is preferably applicable to the present invention when it is composed of a compound semiconductor containing no Al at the contacting interface with the oxide transparent electrode layer and having a band gap energy of less than 1.42 eV. Possible reasons why the contact resistance of the oxide transparent electrode layer can be reduced by using such electrode contact layer are as follows:
(1) the oxide transparent electrode layer of a conventional light-emitting device for example was formed so as to be brought into contact with an AlGaAs current spreading layer, where AlAs alloy composition had to be considerably high in order to ensure a sufficient level of transparency of such current spreading layer. The AlGaAs alloy having a large AlAs alloy composition is, however, very likely to be oxidized since it contains Al in a high concentration, so that formation of the oxide transparent electrode layer allows oxygen contained therein to react with the Al component in the AlGaAs current spreading layer to thereby form an oxide layer having a high resistivity; and
(2) the AlGaAs alloy having a large AlAs alloy composition generally used for the current spreading layer has, although variable with the alloy composition, a band gap energy of as high as 2.02 to 2.13 eV, which is disadvantageous in that achieving ohmic contact or a low-resistivity contact nearly equivalent thereto (typically 10xe2x88x924 xcexa9xc2x7cm or below; these statuses of contact will generally be expressed as having ohmic contact status hereinafter). A problem similar to the case with AlGaAs may arise also when the oxide transparent electrode layer is directly stacked on the AlGaInP cladding layer without using the AlGaAs layer, since the band gap energy thereof is as high as 2.3 to 2.35 eV and Al is contained therein.
However by composing the electrode contact layer as described in the above at the contacting interface with the oxide transparent electrode layer, ohmic contact can readily be attained since the high-resistivity oxide layer is unlikely to be formed due to absence of Al in the contacting interface with the electrode contact layer, and since the band gap energy is small (less than 1.42 eV; which is typically 0.75 eV for In0.5Ga0.5As). This is successful in reducing contact resistance of the transparent electrode layer to a considerable degree. The compound semiconductor composing the electrode contact layer at the contacting interface with the oxide transparent electrode layer can more specifically be expressed as InxGa1-xAs (0 less than xxe2x89xa61).
Materials composing the oxide transparent electrode layer can be those mainly comprising tin oxide (SnO2) or indium oxide (In2O3). More specifically, ITO film having a high electric conductivity is preferably used for the oxide transparent electrode layer in the present invention. ITO film is an indium oxide film doped with tin oxide, where controlling the content of tin oxide within a range from 1 to 9 wt % can sufficiently suppress the resistivity of the electrode layer to as low as 5xc3x9710xe2x88x924 xcexa9xc2x7cm or less. Besides the ITO electrode layer, zinc oxide (ZnO) again having a high electric conductivity is applicable to the present invention. Still other materials available for the oxide transparent electrode layer include tin oxide doped with antimony oxide (so-called Nesa), Cd2SnO4, Zn2SnO4, ZnSnO3, MgIn2O4, CdSb2O6 doped with yttrium (Y) oxide, and GaInO3 doped with tin oxide. In short, the oxide transparent electrode layer can contain at least any one of indium, tin and zinc.
These oxide transparent electrode layers can be formed by known vapor phase film growth processes, examples of which include chemical vapor deposition process (CVD); physical vapor deposition (PVD) processes such as sputtering and vacuum evaporation; and molecular beam epitaxy (MBE) process. For example, ITO electrode layer and ZnO electrode layer can be produced by RF sputtering or vacuum evaporation, and Nesa film can be produced by CVD process. In place of these vapor-phase growth processes, it is also allowable to employ sol-gel process or other processes for the film growth.
The oxide transparent electrode layer can be formed so as to cover the entire surface of the main surface of the light emitting layer section. Such constitution is advantageous in that allowing the oxide transparent electrode layer to function as the current spreading layer, so that it is no more necessary to form a thick current spreading layer comprising a compound semiconductor as has previously been used, or the thickness thereof, even when it is to be formed, can considerably be reduced, which contributes cost reduction through simplifying the processes and is fairly beneficial from an industrial viewpoint. On the other hand, the thickness of the electrode contact layer need not be so thick provided that it is sufficient for achieving ohmic contact. More specifically for the case where a compound semiconductor composes the electrode contact layer, it is desirable to ensure a thickness which is not causative of shifting of the band gap energy from that of the bulk crystal after thinning, where a thickness of 0.001 xcexcm or above will be sufficient (when In-containing GaAs, such as InxGa1-xAs, is used). This advantageously shortens the inter-layer distance between the oxide transparent electrode layer and the light emitting layer section as compared with that in the conventional light-emitting device, and results in a larger reducing effect of the series resistance. It should now be noted that an excessively large thickness of the electrode contact layer comprising InxGa1-xAs undesirably increases light absorption by the electrode contact layer and thus lowers the light extraction efficiency, so that the thickness is preferably adjusted to 0.02 xcexcm or below.
The light emitting layer section composed of (AlxGa1-x)yIn1-yP (where 0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61) or InxGayAl1-x-yN (where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61 and x+yxe2x89xa61) contains Al for most cases and thus raises an issue of oxidative degradation, but adopting a constitution in which the oxide transparent electrode layer covers the entire surface thereof will be advantageous in that allowing such oxide transparent electrode layer to function as a passivation film for the light emitting layer section.
Although InxGa1-xAs is a compound semiconductor possibly having, depending on the alloy composition thereof, a little larger difference in lattice constant as compared with that of a compound semiconductor composing the light emitting layer section, adverse effect due to such lattice mismatching will be suppressed to a relatively small degree if it is formed as a thin layer having a thickness of approx. 0.001 to 0.02 xcexcm, both ends inclusive. The electrode contact layer can thus be formed using this compound semiconductor.
When the electrode contact layer is formed using a compound semiconductor layer so as to directly contact with the oxide transparent electrode layer, it is preferable to use a compound semiconductor having a band gap energy of less than 1.42 eV at the contacting interface with the oxide transparent electrode layer in view of forming a desirable ohmic contact therewith, as described in the above. Since adverse effect of lattice mismatching can be relieved by the thinning as described in the above, it is also allowable to use InP, InAs, GaSb, InSb or alloy thereof, besides InGaAs.
The light emitting layer section comprising (AlxGa1-x)yIn1-yP or InxGayAl1-x-yN can be composed as a double heterostructure which comprises a first conductivity type cladding layer, an active layer and a second conductivitytype cladding layer stacked in this order, which layers being respectively composed of the above-described (AlxGa1-x)yIn1-yP or InxGayAl1-x-yN. Energy barrier ascribable to difference in band gaps with the cladding layers formed on both sides of the active layer can effectively confine injected holes and electrons within a narrow active layer so as to promote recombination thereof, so that an extremely high emission efficiency can be attained. Further adjustment of component of the active layer can provide a wide range of light emission, where the former covers green to red region (peak emission wavelength falls within a range from 520 to 670 nm), and the latter covers ultraviolet to red region (peak emission wavelength falls within a range from 300 to 700 nm).
In the foregoing constitution, the electrode contact layer can be formed between the oxide transparent electrode layer and either of the first conductivity type cladding layer and second conductivity type cladding layer. For a typical case where only a main surface on one side of the light emitting layer section having a double heterostructure is used as a light extraction surface, the electrode contact layer is first formed between the cladding layer resides on that side and the oxide transparent electrode layer so as to contact with such oxide transparent electrode layer, and then the oxide transparent electrode layer can be formed. On the other hand, for a typical case where the main surfaces on both sides of the light emitting layer section are used as light extraction surfaces, the oxide transparent electrodes can be formed respectively for these cladding layers on both sides, and between the oxide transparent electrode and the cladding layer, the electrode contact layer can be formed so as to contact with such oxide transparent electrode layer.
It is also allowable to form an intermediate layer between the electrode contact layer and either of the cladding layers facing to the electrode contact layer, which is either of the first conductivity type cladding layer and the second conductivity type cladding layer, where the intermediate layer has an intermediate band gap energy between those of the electrode contact layer and such cladding layers. In order to enhance the carrier confinement efficiency into the active layer to thereby raise the internal quantum efficiency, it is necessary for the double heterostructured, light emitting layer section to have a barrier height between the cladding layer and the active layer raised to a certain level. As shown in a schematic band chart in FIG. 12 (Ec represents an energy level of the bottom of the conduction band, and Ev represents that of the top of the valence band), direct contacting of such cladding layer (AlGaInP layer, for example) with the electrode contact layer (InGaAs layer, for example) may sometimes result in generation of a relatively high hetero barrier therebetween due to contacting-induced bending of the energy band. The barrier height xcex94E increases as the band-end discontinuity value between the cladding layer and the electrode contact layer increases, which is more likely to block carrier motion, in particular motion of holes having a larger effective mass. In a typical case using a metal electrode, coverage with such metal electrode over the entire surface of the cladding layer will prevent extracting of the light, so that the electrode must be formed only with a partial coverage. In this case, some strategy will be necessary to promote outward current spreading in the in-plane direction of the electrode in order to improve the light extraction efficiency. While many cases using the metal electrode employ an electrode contact layer typically composed of GaAs between the light emitting layer section and the metal electrode, it is more beneficial for the case where the metal electrode is used that a properly high barrier is formed between the electrode contact layer and the light emitting layer section in terms of promoting current spreading in the in-plane direction by virtue of carrier blocking effect expected from such barrier. However, formation of high barrier consequently results in increase in series resistance.
In contrast, it is almost unnecessary for the case using the ITO electrode layer to consider the carrier blocking effect expected from the barrier, since the ITO transparent electrode per se has a considerably high current spreading property. Still another advantage of using the ITO electrode layer is such that the area from which the light can be extracted increases to a considerable degree as compared with the case using the metal electrode. Inserting now the intermediate layer between the electrode contact layer and the cladding layer as shown in FIG. 13, which intermediate layer having an intermediate band gap energy between those of the electrode contact layer and such cladding layer, will successfully reduce band-edge discontinuity value between the electrode contact layer and the intermediate layer, and between the intermediate layer and the cladding layer, which consequently lowers the barrier heights xcex94E respectively formed therebetween. This eventually reduces the series resistance, and makes it possible to achieve a sufficiently high luminous intensity even under a low drive voltage.
Effect of using the intermediate layer will be eminent in particular when the double heterostructured, light emitting layer section is formed using AlGaInP which has a relatively good lattice matching with the In-containing GaAs which composes the electrode contact layer. For this case, the intermediate layer having an intermediate band gap energy between those of the electrode contact layer and the cladding layer is preferably formed while containing at least one of AlGaAs layer, GaInP layer and AlGaInP layer (having a composition adjusted so as to suppress the band gap energy lower than that of the cladding layer), which is exemplified by such that including an AlGaAs layer. The intermediate layer is also applicable to any light emitting layer sections other than that described in the above, such as a double heterostructured, light emitting layer section typically composed of InxGayAl1-x-y. For this case, the intermediate layer is preferably such that including an InGaAlN layer (having a composition adjusted so as to suppress the band gap energy than that of the cladding layer).
Next, the method for manufacturing light-emitting device according to the present invention is such that manufacturing a light-emitting device having a light emitting layer section which is composed of (AlxGa1-x)yIn1-yP (where 0xe2x89xa6xxe2x89xa61and 0xe2x89xa6yxe2x89xa61) as having a double heterostructure which comprises a first conductivity type cladding layer, an active layer and a second conductivity type cladding layer stacked in this order; and having an ITO electrode layer for applying drive voltage for light emission to such light emitting layer section provided on either side of the first conductivity type cladding layer and the second conductivity type cladding layer; which comprises the steps of:
forming a GaAs layer on the light emitting layer section so that occupied areas and unoccupied areas for the GaAs layer are arranged in a mixed manner;
forming the ITO transparent conductive layer so as to contact with the GaAs layer; and
annealing the ITO electrode layer so as to allow In contained therein to diffuse into the GaAs layer to thereby convert such GaAs layer into an In-containing electrode contact layer.
In the method for manufacturing light-emitting device of the present invention, a GaAs layer is formed on the light emitting layer section which is composed of AlGaInP, and an ITO electrode layer is formed so as to contact with the GaAs layer. The light emitting layer section is typically composed of a III-V group compound semiconductor, and can typically be formed by known MOVPE process together with the GaAs layer formed thereon (where interposition of any other lattice-matched layer permissible). The GaAs layer can very easily establish lattice matching with the AlGaInP light emitting layer section, and can be formed with better uniformity and continuity as compared with those of an InGaAs layer directly formed thereon by the epitaxial growth process.
The ITO electrode layer is formed on the GaAs layer, and is then annealed so as to diffuse In from the ITO electrode layer towards the GaAs layer to thereby convert it into the electrode contact layer. Thus annealed electrode contact layer which is composed of In-containing GaAs will never have an excessive In content, and can effectively prevent quality degradation such as lowered luminous intensity. Since the lattice matching between the GaAs layer and the light emitting layer section will be especially desirable when the light emitting layer section is composed of (AlxGa1-x)yIn1-yP (where 0xe2x89xa6xxe2x89xa61 and 0.45xe2x89xa6yxe2x89xa60.55), so that it is preferable to form the light emitting layer section (cladding layer or active layer) while setting the alloy composition xe2x80x9cyxe2x80x9d within the above range.
The foregoing annealing is preferably carried out so that the electrode contact layer will have a distribution of the In concentration along the thickness-wise direction thereof such that continuously decreasing as the distance from the ITO electrode layer increases (that is, creating an In concentration gradient) as shown by line {circle around (1)} in FIG. 16. Such constitution is attainable by allowing under annealing In to spread from the ITO side to the electrode contact layer side.
In the light-emitting device of the present invention in this case, the light emitting layer section is composed of (AlxGa1-x)yIn1-yP (where 0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa61) as having a double heterostructure which comprises the first conductivity type cladding layer, the active layer and the second conductivity type cladding layer stacked in this order, an ITO electrode layer as the oxide transparent electrode for applying drive voltage for light emission to the light emitting layer section is provided on either side of the first conductivity type cladding layer and the second conductivity type cladding layer so that the light from the light emitting layer section can be extracted through such ITO electrode layer, the electrode contact layer composed of In-containing GaAs is formed so as to contact with such ITO electrode layer, and the electrode contact layer has a distribution of the In concentration along the thickness-wise direction thereof such that continuously decreasing as the distance from the ITO electrode layer increases. This means that, on the side closer to the light emitting layer section which is composed of AlGaInP, the electrode contact layer has a lower In concentration, in other words, that difference in the lattice constant with that of the light emitting layer section decreases. Formation of the electrode contact layer having such In concentration distribution is beneficial in that further improving lattice matching with the light emitting layer section. Excessively high annealing temperature or excessively long annealing time will result in excessive In diffusion from the ITO electrode layer, which makes the In concentration distribution almost constant over the thickness range of the electrode contact layer as indicated by line {circle around (3)} in FIG. 16, and fails in obtaining the above-described effect (conversely, excessively low annealing temperature or excessively short annealing time will result in shortage of the In concentration in the electrode contact layer as indicated by line {circle around (2)} in FIG. 16).
Assuming now in FIG. 16 that CA is the In concentration in the vicinity of the interface with the ITO electrode layer, and CB is the In concentration in the vicinity of the interface opposite thereto, a value of CB/CA is preferably adjusted to 0.8 or below, and it is preferable to carry out the foregoing annealing so as to obtain such form of In concentration distribution. A value of CB/CA exceeding 0.8 will result in only an insufficient improving effect of lattice matching with the light emitting layer section based on In concentration gradient. Compositional depth profile (In or Ga concentration distribution) of the electrode contact layer can be measured by known surface analytical technique such as secondary ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS), while gradually etching the layer in the depth-wise direction.
The electrode contact layer preferably has an In concentration in the vicinity of the interface with the ITO electrode layer, as being expressed by an atomic ratio of the In concentration to the total concentration of In and Ga, of 0.1 to 0.6, and it is preferable to carry out the foregoing annealing so as to obtain such In concentration. The In concentration according to the above definition less than 0.1 will result in only an insufficient effect of reducing contact resistance of the electrode contact layer, and that exceeding 0.6 will result in serious deterioration such as lowered luminous intensity due to lattice mismatching between the electrode contact layer and the light emitting layer section. It is to be noted that as far as the electrode contact layer can keep the In concentration CA in the vicinity of the interface with the ITO electrode layer typically within the above preferable range (0.1 to 0.6) as being expressed by an atomic ratio of the In concentration to the total concentration of In and Ga, there will be no problem if the In concentration CB in the vicinity of the interface opposite to that facing to the ITO electrode layer has a value of zero, which is typified by a constitution in which the electrode contact layer has an InGaAs layer formed on the ITO electrode layer side, and has a GaAs layer formed on the opposite side.
ITO refers to indium oxide doped with tin oxide as described in the above. Formation of the ITO electrode layer on the GaAs layer, and annealing thereof within a proper temperature range readily provides the electrode contact layer having an In concentration within the foregoing preferable range. The annealing is also beneficial in further reducing the electrical resistivity of the ITO electrode layer. The annealing is preferably carried out within a range from 600xc2x0 C. to 750xc2x0 C. The annealing temperature exceeding 750xc2x0 C. tends to excessively accelerate the In diffusion into the GaAs layer, which often makes the In concentration in the electrode contact layer excessive. It is also anticipated that this makes it difficult to obtain the In concentration gradient such that being inclined along the thickness-wise direction of the electrode contact layer due to saturation of the In concentration. Both situations degrade the lattice matching between the electrode contact layer and the light emitting layer section. An excessive diffusion of In into the GaAs layer inevitably raises electrical resistivity of the electrode since In in the ITO electrode layer is exhausted around the contact area with the electrode contact layer. Still another problem of excessively high annealing temperature is that series resistance of the device is more likely to increase, which is ascribable to promoted oxidation of GaAs layer by oxygen spreaded from ITO. Both situations result in malfunction such that the light-emitting device cannot be driven at a proper voltage. An extremely high annealing temperature can even worsen the electrical resistivity of the ITO electrode layer. On the contrary, the annealing temperature lower than 600xc2x0 C. will excessively reduce the diffusion rate of In into the GaAs layer, so that a longer time will be necessary to sufficiently lower the contact resistance of the electrode contact layer, which considerably ruin production efficiency.
The annealing time is preferably set within a range from 5 to 120 seconds. The annealing time longer than 120 seconds will tends to excessively raise the amount of In diffusion into the GaAs layer in particular for the case where the annealing temperature is set close to the upper limit. It is, however, allowable to set the annealing time longer than 120 seconds (typically to as long as approx. 300 seconds) when the annealing temperature is set relatively low. On the other hand, the annealing time shorter than 5 seconds will make it difficult to obtain the electrode contact layer having a sufficiently low contact resistance due to shortage in the amount of In diffusion into the GaAs layer.